The incident happened at the Harlem Station in Forest Park, Illinois, west ofChicago. One train was standing at the station at the time of the accident at 7:45 a.m. local time, according to CTA spokeswoman Catherine Hosinski. The cause of the accident is under investigation.

Thirty-three people were transported to nine area hospitals with what appeared to be non-serious injuries, Hosinski said. One of those requesting medical attention was a train operator.

Service on the line was shut down for a couple of hours and the station where the collision occurred remained closed after service was restored, the CTA said.